1. Field of the Invention
The present invention relates to the purification of crude wet-process phosphoric acid by removal of contaminating amounts of heavy metal values therefrom, notably molybdenum values, and, more especially, relates to the preparation of food grade phosphoric acid utilizing such purification.
2. Description of the Prior Art
Phosphoric acid is of course widely used in a great number of food and beverage products, and the demand therefor is constantly increasing. Furthermore, the phosphoric acid used in the food industry was until recently that acid prepared by thermal process. Such acid is known to contain only very small quantities of impurities, particularly as regards impurities which would adversely detract from food and beverage products.
Thermal processing is, however, quite expensive. Thus, the art has looked to the use of wet-process phosphoric acid for foodstuff applications, which is much cheaper to manufacture.
Unfortunately, wet-process phosphoric acid contains many impurities and particularly metallic elements such as lead, mercury and arsenic, which make it impossible to use such acid in the food industry without having subjected same to upstream purification.
Food grade acids must indeed be manufactured in accordance with very specific and strict standards, governmental and otherwise. In particular, commercial food grade acids, namely, those theoretically containing 75 to 85% of H.sub.2 PO.sub.4, or approximately 54 to 62% of P.sub.2 O.sub.5, are necessarily subjected to the "heavy metal test".
The purpose of such test is to determine the heavy metal content of the acids. The best known of the heavy metals include, particularly, silver, mercury, lead, copper, cadmium and bismuth.
The test consists of determining the color of a sample of phosphoric acid when treated with H.sub.2 S at a specific pH value (between 3 and 4). The resulting color is next compared with those of control solutions containing known amounts of Pb.sup.2+ and the results are extrapolated into heavy metals expressed as lead. Also, for the test to be significant, it has to be conducted upon those acids which have, during the manufacture thereof, been subjected to treatment with H.sub.2 S or a sulfide in a highly acid medium (neat acid), such that the arsenic sulfide is precipitated and can then be filtered off. Without the immediately aforesaid, the arsenic sulfide would impart a yellow tint, which would falsify the results of the test. The aforesaid removal of arsenic values is carried out to such extent that commercial strength acid results.
For food grade acids and acids at commercial strength, as aforesaid, the amount of contaminating metal values therein must be less than 10 ppm.
Now, in the case of most wet-process phosphoric acids, even after the known purifications, and especially those entailing liquid/liquid extraction with an organic solvent and the contacting with hydrogen sulfide at commercial strength, it is impossible to develop a heavy metal test which will comply with the foregoing standard.
It has now been determined by applicants, however, that the fact that the noted test produced a value of over approximately 10 ppm is essentially due to the molybdenum, which is present in substantial quantities in most wet-process phosphoric acids, and particularly even in those purified by treatment with an organic solvent. The action of H.sub.2 S on molybdenum salts indeed produces a yellow color, with the result that the test Will not comply with the standard.